Third experiment homes in on neutrino mixing angle

Taking seventh place in our top 10 breakthroughs for 2011 were physicists working on the Tokai-to-Kamioka (T2K) experiment in Japan, who were the first to measure the rate at which muon neutrinos change into electron neutrinos (and then back into muon neutrinos) as they travel hundreds of kilometres through the Earth. This neutrino oscillation was then observed by scientists on the similar MINOS experiment in the US, with some degree of agreement between the two values. Both experiments provided important new information about the physics of neutrinos, which is by no means settled.

Neutrinos come in three “flavour” states – electron, muon and tau. However, physicists also believe that neutrinos can be described in terms of combinations of three mass states – m1, m2 and m3. Interference between these mass states gives rise to the observed oscillations of neutrino flavour.

Although physicists have measured many of the parameters that describe this flavour/mass system, one crucial value remains unclear. This is the “mixing angle” θ13, which is a measure of how the m1 and m3 mass states are combined within the flavour states.

T2K and MINOS have both given preliminary values for θ13 that are in rough agreement, and now a third experiment – Double Chooz in France – has also determined θ13 (what is actually measured is sin22θ13).

Double Chooz took a different approach by looking at electron antineutrinos that are produced in two nuclear reactors and detected about 1 km away after travelling through solid rock. The electron antineutrinos are expected to oscillate to either muon or tau antineutrinos, and the rate at which electron antineutrinos vanish from the beam due to oscillation is determined by θ13 and one other parameter that is well known.

The experiment was run for 101 days and the number of electron antineutrinos that should have been detected was calculated to be 4344. Instead, the physicists only saw about 4100 events in the detector.

Double Chooz found that sin22θ13 is about 0.086, whereas T2K suggests a value of about 0.11 and MINOS gives 0.04. All of these figures have large uncertainties associated with them and cannot be seen as definitive measurements of the mixing angle. What’s becoming clear, however, is that θ13 is not zero.

Once θ13 has been determined, the next challenge for physicists will be to work out if there are differences between the oscillations experienced by neutrinos and antineutrinos. The discovery of such an asymmetry could shed light on why there is much more matter than antimatter in the universe.